Metal-oxide-semiconductor (MOS) field-effect transistors (FET) have been a dominating technology for integrated circuits. A MOSFET can work in three regions, depending on gate voltage Vg and source-drain voltage Vds. These three regions include linear, saturation, and sub-threshold regions. The sub-threshold region is a region wherein gate voltage Vg is smaller than threshold voltage Vt. The sub-threshold swing represents the easiness of switching the transistor current off and is an important factor in determining the speed and power of a MOS device. The sub-threshold swing can be expressed as a function of m*kT/q, wherein m is a parameter related to capacitance. The sub-threshold swing of conventional MOS devices has a limit of about 60 mV/decade (kT/q) at room temperature, which in turn sets a limit for further scaling of operation voltage VDD and threshold voltage Vt. This limitation is due to the drift-diffusion transport mechanism of carriers. For this reason, existing MOS devices typically cannot switch faster than 60 mV/decade at room temperatures. The 60 mV/decade sub-threshold swing limit also applies to FinFET or ultra-thin body MOSFET on silicon-on-insulator (SOI) devices. Therefore, with better gate control over the channel, a newer ultra-thin body MOSFET on SOI or a finFET can achieve a sub-threshold swing close to, but not below, the limit of 60 mV/decade. With such a limitation, faster switching at low operation voltages for future nanometer devices is challenging to achieve.
The tunnel field-effect transistor (TFET) is a newer type of transistor. TFETs switch by modulating quantum tunneling through a barrier. Because of this, TFETs are not limited by the thermal Maxwell-Boltzmann tail of carriers, which limits MOSFET subthreshold swing to about 60 mV/decade of current at room temperature.